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Changes in dry weights, reduced N, nitrate, and nitrate reductase activity of various plant parts of the above ground vegetation (stover) and ears of field grown maize were measured at intervals between anthesis and grain maturity. Nonstructural carbohydrate contents were also measured in some instances. Changes in dry weight and reduced N content were used to approximate net in situ photosynthetic and nitrate assimilation activities and to determine whether the availability of photosynthate or reduced N was limiting grain production.Of the five hybrids studied, all showed extensive remobilization (loss) of reduced N from the stover during grain development. This loss of stover N was initiated by 18 to 21 days after anthesis. Most of this loss of N (about 70%) was from the leaves. In contrast, three of the five hybrids had more vegetative dry weight at grain maturity than at anthesis, while the loss of stover dry weight by the other two hybrids was negligible. By 42 days after anthesis when the bulk of the ear weight had been acquired, the average gain in stover dry weight for the five hybrids was 12% while the loss of stover reduced N was 28%. Where measured, the increase in stover dry weight was largely due to deposition of carbohydrates in the stalk. These results show that the photosynthetic capacity was adequate while nitrate reduction capacity was inadequate for ear demands. The changes in the rate of accumulation of dry weight and reduced N by the ear indicated that the rate of supply of reduced N to the ear could have limited ear development for one of the five hybrids. The dry weight and carbohydrate (where measured) accumulation in the vegetation during the first 42 days after anthesis infers that the rate of supply of photosynthate to the ear was probably not a limiting factor for any of the five hybrids.The maximum remobilization of stover N during grain development was 1.8 g N plant(-1) for the genotypes examined, while the amount of reduced N accumulated by the grain varied from 1 to 5 g plant(-1). The amount of newly reduced N (nitrate reduced after anthesis) provided from 48 to 72% of the total N accumulated by the ear. The relative amounts of newly reduced N and remobilized N vary with genotype and environment. With respect to insuring high productivity, it was concluded that there is more flexibility in the system (genotype and environment) for increasing the supply of newly reduced N than in enhancing the remobilization of vegetative N.

Availability of Reduced N and Carbohydrates for Ear Development of Maize